What is anode mud?

On passing the electric current pure metal from the anode dissolves and gets deposited at the cathode. 

The soluble impurities go into the solution while insoluble impurities or less electropositive impurities settle down below the anode as anode mud or sludge.

Final answer : The correct answer is option C.  Impurities collected at anode in electrolysis during purification of metals